Previously, carbon materials have been widely used as electrode materials, heating elements, structural materials, heat insulating materials, heat-resistant sealing materials, X-ray parts, and the like because of their excellent heat resistance, chemical resistance, and electrical conductivity. In particular, sheet-form carbon films are expected to be applicable to important uses as industrial materials as described above, and have been extensively studied in recent years.
Processes for preparing carbon films include a process in which a carbon film is prepared from natural graphite, a process in which a carbon film is prepared by the high-temperature decomposition deposition of a hydrocarbon in a gas phase, and a process in which a carbon film is prepared by treating an organic material or a carbonaceous material at an elevated temperature. Carbon films obtained from these processes are presently being applied to fields which utilize their excellent characteristics, such as heat resistance, chemical resistance, and electrical conductivity.
Among these processes for preparing carbon films, processes for obtaining a carbon film by heat-decomposing a polymer film have been extensively studied because of their simplicity, and many attempts have been made to improve the process as disclosed in JP-A-53-139676 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-60-11215 (corresponding to U.S. Pat. No. 4,599,193), JP-A-60-60-235709, JP-A-62-91414 (corresponding to U.S. Pat. No. 4,915,984) and JP-A-1-105199 (corresponding to U.S. Pat. No. 4,842,665). Among the processes of the type described above, a process in which an aromatic polyimide film is used as the polymer film, which is heat-decomposed by heat-treating it at an elevated temperature to obtain a carbon film, is described in, for example, JP-B-64-12305 (the term "JP-B" as used herein means an "examined Japanese patent publication"). A process in which a polymer film is stretched and then carbonized is disclosed in, for example, JP-B-1-48204 (corresponding to U.S. Pat. Nos. 4,626,588 and 4,791,177).
Conventional carbon films exhibit satisfactory electrical conductivity or mechanical properties. However, a carbon film which exhibits both improved electrical conductivity and mechanical properties has not yet been produced. A carbon film having a tensile strength of at least 15 Kgf/mm.sup.2, a tensile modulus of elasticity of at least 5,000 Kgf/mm.sup.2 and an electric conductivity of at least 200 S/cm (S=mho), has not been previously produced.
For example, a graphite layer should be orderly arranged in the film by heat-treating the film at a temperature of at least 3000.degree. C. in an oxygen-free atmosphere under high pressure to obtain a carbon film which exhibits excellent electrical conductivity. However, it is difficult to manufacture an apparatus which is to be operated at such a high temperature. In addition, carbon films obtained by such a high-temperature treatment are generally brittle, and do not always exhibit excellent mechanical properties.
Further, there is no known process for preparing a carbon film having a tensile strength of at least 15 Kgf/mm.sup.2, a tensile modulus of elasticity of at least 5,000 Kgf/mm.sup.2 and an electric conductivity of at least 200 S/cm using aromatic polyimides as a starting material.